Semiconductor devices typically require materials that are relatively free of contaminants. There are many possible contaminants, and mobile ions, such as sodium ions, are known to be a common contaminant in the dielectrics used in semiconductor device fabrication. For example, such contaminants may be present in the silicon oxide commonly used as a gate oxide in silicon field effect transistors. Such oxides are typically grown by thermal oxidation of the underlying silicon and are generally very thin, for example, less than 100 nm. Mobile ions are known to be undesirable in gate oxides because they tend to cause shifts in device operating voltages. Accordingly, methods have been developed to measure mobile ion concentrations in such oxides.
The conventional method of measuring mobile ion concentration in an oxide layer typically includes periodically varying the voltage, i.e., "sweeping" the voltage, applied to electrodes on opposing sides of the oxide layer and measuring the displacement current that passes through the oxide. Such current typically rises suddenly near zero volts, typically within a few tens of millivolts, and is not typically dependent on the oxide thickness. It is generally believed by workers in the art that such current-versus-voltage behavior arises as weak bonds which have held the mobile ions near the electrode/oxide interface are broken. As a result, a voltage sweep between only very small voltage limits, typically .+-.5 volts or less, produces an observable peak displacement current very close to zero volts. Higher voltages have not been thought to be needed for oxides and in many prior situations would not be practical because dielectric breakdown might occur for the very thin thermal oxides.
The above described method is generally referred to as the Triangular Voltage Sweep ("TVS") method, and is described, for example, in a paper entitled "The Application of Triangular Voltage Sweep Method To Mobile Charge Studies In MOS Structure," J. Electrochem. Soc., Vol. 118, No. 4, April, 1971, pp. 601-609.
As is well known, dielectrics other than relatively thin, thermally grown oxides are also now commonly used. For example, relatively thick oxides deposited by low temperature chemical vapor deposition techniques are commonly used in semiconductor device fabrication as interlevel dielectrics between two levels of interconnecting metal patterns. Such deposited oxides also are susceptible to mobile ion contamination, which can adversely affect device characteristics as it does for thermally grown gate oxides. However, measurement of the mobile ion concentration in deposited oxides has been more difficult than has the corresponding measurement for thermal oxides.
Difficulties arise for several reasons including the thickness of the deposited oxides, e.g., typically substantially greater than 100 nm, the low temperature heat treatment the deposited oxide generally encounters, and the disordered nature of the dielectric. The deposited oxides are generally less dense than are the thermal oxides and may be considered more disordered than the thermal oxides. The term "disorder" may seem somewhat misdescriptive as applied to an amorphous material such as a glass; however, it is believed that the deposited oxides have more dangling chemical bonds than do the thermal oxides. For this reason, they typically are considered to be more disordered.
Prior attempts to measure mobile ion contamination in thick deposited oxides, using the conventional TVS method have been unsuccessful, i.e., have resulted in no displacement current peaks in the normal .+-.5 volts range, even with oxide layers which were later found to have significantly deleterious amounts of mobile ion contamination.